Can't defrag under XP-SP2

Can't seem to defrag hard drive under Win XP - SP2. Some error message abt the MS mgmt console comes up instead. Was fine before SP2 installation. Have plenty of free space so that's not the problem. Any ideas?

Tried the blackviper solution (very useful one btw) but still couldn't defrag. Decided to just uninstall SP2, and just use the hotfixes from MS support. HD defrags fine now but can't reinstall SP2 again, guess I'll just have to live with it since it's running stable. Thanks for your help!

Before you uninstalled SP2 did you try this?
Go to the Control Panel and open Administrative Tools; double-click on Services, scroll down to Windows Installer, right-click on it and select Properties. Change Startup Type to Automatic. Reboot and try Defrag again.